The effects of molecular orientation on the physical aging and mobility ofpolycarbonate-solid state NMR and dynamic mechanical analysis

Solid-state NMR and dynamic mechanical (DR;LA) measurements were performed on a series of uniaxially hot-drawn bisphenol-A polycarbonate samples in or der to determine the effects of stretching on the structure, mobility, and local orientation environment. Proton spin-lattice relaxation times, H-1 T- 1 rho, for the phenylene carbon protons were fitted to a biexponential deca y function, and both the long and short relaxation times initially increase d with stretching. Intensity data indicated an increase in the number of sh ort relaxation time protons and a decrease in the number of long relaxation protons with orientation. Similarly, DMA spectra showed that the beta -rel axation strength also increased with drawing, which implied an increase in the number of localized segmental relaxations. It is theorized that the lon g and short H-1 T-1 rho relate to protons within tightly packed "cooperativ e domains," and to those with greater localized free-volume, respectively. Stretching is known to distort the free-volume distribution, causing a decr ease in the mean free-volume but an increase in the number of larger, more elliptical holes. This is expected to cause a decrease in the alpha -transi tion mobility (due to larger cooperative domains) and an increase in the be ta -mobility (due to the increase in the number of beta -relaxing segments associated with the larger free-volume holes). These predictions are consis tent with results recently reported by Shelby and Wilkes on the physical ag ing and creep behavior of these samples (M. D. Shelby & G. L. Wilkes, Polym er 1998, 39, 6767; M. D. Shelby & G. L. Wilkes, J Polym Sci Part B: Polym P hys 1998, 36, 2111). (C) 2000 John Wiley & Sons, Inc.